In a phone interview with Reuters news agency from Syria on Tuesday, SDF spokesman Talal Silo said the operation started on Monday and the fighting would be "fierce because ISIL will die to defend their so-called capital".

The Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group captured the city from rebel groups in 2014 and has since used it as their home base.

"The coalition has a big role in the success of the operations. In addition to warplanes, there are coalition forces working side by side with the Syrian Democratic Forces," Silo said.

The UK-based Syrian Observatory for Human Rights said the SDF attacked the eastern edge of Raqqa and a military base on the northern outskirts of the city on Tuesday.

The attack on the al-Mashlab district and on the Division 17 base around 1km to the north of the city centre followed heavy overnight air raids, the observatory said.

The SDF has been working to encircle Raqqa since November in an offensive backed by the US-led coalition that is also fighting ISIL in Iraq, where the group is on the brink of defeat in the city of Mosul.

The US-led coalition has said 3,000 to 4,000 ISIL fighters are thought to be holed up in Raqqa city, where they have erected defences against the anticipated assault.

The US said on Tuesday it had started distributing arms to the YPG to help take Raqqa, part of a plan that has angered NATO-ally Turkey, which is worried by growing Kurdish influence in northern Syria.

Turkey's Prime Minister Binali Yildirim warned on Tuesday that the country will retaliate immediately if operations in Raqqa presents a threat to Turkey.

Coalition air strikes

The observatory reported that the death toll of suspected US-led coalition air attacks that allegedly targeted boats carrying people who tried to flee the fighting in Raqqa city on Monday has risen to 21 civilians.

Raqqa is Being Slaughtered Silently, an activist group in Raqqa, said people attempted to flee after previous coalition air raids targeted the old and new bridge areas near Raqqa city.

After the two bridges were bombed most of the civilians started to use the boats to cross from the city side to the countryside.

Government warplanes struck Tafas town in the northwestern suburbs of Deraa city on Monday.

Deraa, the city where the Syrian war began in March 2011 with anti-government protests, is part of the four "de-escalation zones" agreed on earlier this month during ceasefire talks in Astana, Kazakhstan.

The Observatory also said that at least 22 barrel bombs were dropped by government helicopters on the Deraa al-Balad neighbourhood while 35 surface-to-surface missiles were launched by government forces on Monday.

In the past 48 hours, at least 110 air attacks have targeted Deraa, the Observatory added.

Last week, rebels attacked government positions in the Manshiyeh neighbourhood in Deraa city.

At least 31 rebel fighters and government forces were killed in the recent fighting.

Syria's state news agency SANA reported on Monday that at least seven civilians were killed in rocket attacks by the rebels in a Deraa suburb just 4km east of Deraa city.